A problem exists with stray fields. Magnetic fields may be used to provide power and to communicate with remote identification tags. These fields are generated by currents flowing in coils. To operate tags at distances considered useful, a substantial field at the tag is needed, and providing this field can lead to relatively significant stray magnetic fields beyond the tag. FIG. 1 illustrates this.
The emission of stray magnetic fields is limited by Regulations. Numerous attempts have been made to limit the emission of stray fields without unduly retarding the performance of interrogator/tag communication, but to date they have been considered relatively ineffective.
One form of prior art reduces the stray magnetic field generated by a coil by winding the coil in two oppositely directed halves, as illustrated in FIG. 2. While the field close to either half of the coil is unaffected by the other half, the stray field at a larger distance is reduced due to the fields cancelling each other.
Another form of prior art provides a conducting metal sheet greater than five electromagnetic skin depths thick for reflecting magnetic fields and acting as a screen against oscillating magnetic fields. In this way, the stray field from a coil can be screened by enclosing the coil in a metal box (FIG. 3). However, in many applications such a complete enclosure is not desirable. For example, where powering and communication with tags is needed such a box would require an opening(s) to allow the tag(s) to be passed through the box and near the coil for interrogation purposes. Despite the effectiveness of the metal screening, it has been found that providing openings will enable a relatively large amount of magnetic field to escape, and thus this is not considered desirable (FIG. 4).
Yet another form of prior art is disclosed in Australian Patent application No. 14543/92, filed in the name of Integrated Silicon Design Pty Ltd. In that application, FIGS. 8, 9A, 9B, 9C and 9D disclose various forms of field terminating structures. Effectively, the structures disclosed, at the interrogation frequency, serve to operate as a quarter wavelength long waveguide at the cut-off frequency. It is considered that this structure not suitable for attenuating lower frequency magnetic fields, such as those used in the identification, sorting and location of baggage and other items.